This laboratory has demonstrated that phosphorylation of rabbit skeletal myosin modifies the steady state kinetics of the actomyosin ATPase. These studies will be expanded to include a similar analysis for the soluble subfragments of both cardiac and skeletal myosin, HMM and subfragment one. Methods will be developed to prepare these subfragments without degradation or dissociation of the phosphorylatable light chain, L2. A functional analysis will be undertaken of the components of this light chain kinase extract in order to determine whether there is more than one light chain kinase and whether a particular P-myosin-kinase cimplex is the modifier of the actomyosin ATPase.